Pneumatic rubber tire having at least one component or a rubber composition comprised of a major portion of relatively low Tg high cis 1,4-polybutadiene elastomer (BR) and a minor portion of a relatively high Tg, styrene-rich, styrene/isoprene elastomer (SIR) wherein the Tg of said BR is at least 60xc2x0 C. lower than the Tg of said SIR. Said SIR has a styrene content of at least 30 weight percent. The styrene based units of said SIR are present substantially without large blocks of styrene units. Said BR is a high cis 1,4-polybutadiene having a cis 1,4-content of at least 96 percent and preferably has a branched macrostructure. In one aspect of the invention, said tire component is a tire tread, particularly a circumferential tire tread.
Pneumatic rubber tires are often prepared with a component of a rubber composition which is composed of a blend of elastomers.
For example, a rubber tread component is typically comprised of a blend of elastomers to promote desirable tire tread properties such as, for example, traction, reduction in tread wear and reduced rolling resistance.
Historically, passenger tire treads are commonly composed of rubber blends of a styrene/butadiene copolymer elastomer (SBR), which is usually used to promote traction characteristics for the tire tread, and cis 1,4-polybutadiene rubber which is usually used to promote reduction in tread wear and reduction in rolling resistance for the tire tread.
Such rubber blends are typically SBR-rich in that the SBR is conventionally used as the major elastomer component of the tread rubber composition because tread traction on the road surface is usually a significantly desirable feature for the tire and, also, the SBR in its uncured state is relatively easy to process. Contrarily, the cis 1,4-polybutadiene rubber is conventionally a minor component of the rubber composition because, in its uncured state, it is typically of a relatively high viscosity (high Mooney viscosity) and therefore more difficult to process in rubber processing equipment.
Such SBR-rich tire tread rubber compositions may also sometimes contain minor amounts of one or more other elastomers, in addition to said minor amount of cis 1,4-polybutadiene rubber, such as, for example, cis, 1,4-polyisoprene rubber (particularly natural rubber), isoprene/butadiene copolymer rubber and/or 3,4-polyisoprene rubber, and perhaps trans 1,4-butadiene.
For a tire, it is often desired to optimize one or more of a tire""s treadwear, skid resistance, and rolling resistance by choice of a tire tread""s rubber composition.
However, as it is known to those having skill in such art, an optimization in one of such tire properties by varying the tire tread rubber composition usually diminishes one or more of the other two properties.
Achieving an optimization of all of treadwear, wet skid resistance and rolling resistance by the rubber composition for the tire tread is therefore a challenge.
Accordingly, for high performance tires, an increase in the rate of wear of the tread might be acceptable where very good wet and dry traction of a tire tread is desired. Thus, rubber compositions intended to emphasize very good wet and dry traction for a tire tread may, however, often exhibit a relatively lower resistance to abrasion, higher rolling resistance and lower snow traction and higher brittle point. These formulations usually contain a higher concentration of SBR as a major component of rubber compositions for such tire treads.
A traction promoting elastomer, such as for example styrene/butadiene elastomer, might be used for tread rubber compositions may typically exhibit a relatively high Tg above xe2x88x9250xc2x0 C. such as, for example, Tg within a range of about zero down to about xe2x88x9250xc2x0 C.
However, high Tg polymers typically raise the brittle point of the rubber compositions and such rubber compositions are therefore more susceptible to cracking at cold temperatures. For example, rubber compositions for treads for all-season tires may desirably have a brittle point below xe2x88x9240xc2x0 C.
In contrast to such high Tg styrene/butadiene rubber, an elastomer such as, for example cis 1,4-polybutadiene, used in a tread rubber composition, in combination with such styrene/butadiene rubber, to promote good (relatively low) treadwear might have a much lower Tg, for example, a Tg lower than xe2x88x9290xc2x0 C.
In such rubber composition, the low Tg cis 1,4-polybutadiene rubber is usually used to promote tread wear for a tire tread. However the amount of cis 1,4-polybutadiene is usually limited to a relatively minor component of the elastomers for a tire tread (e.g. usually less than 35 weight percent thereof) because if the cis 1,4-polybutadiene elastomer is present as a major component (e.g. greater than 50 weight percent), the resulting rubber composition typically exhibits a relatively high and, generally unacceptable, mixing and processing viscosity (e.g. a high Mooney ML 1+4 viscosity) without blending a rubber processing oil with the cis 1,4-polybutadiene to reduce its processing viscosity. In addition, levels of cis 1,4-polybutadiene greater than 35 weight percent in treads usually result in unacceptable wet traction.
In such rubber composition, the styrene/butadiene rubber is usually used to promote traction for a tire tread and is typically present as a major component of the elastomers (e.g. greater than 50 weight percent). The styrene content of the styrene/butadiene rubber in commercial tire treads is conventionally in a range of about 10 to about 40 weight percent of the elastomer.
Styrene/butadiene rubber and cis 1,4-polybutadiene rubber are typically compatible and therefore a plot of Tan delta versus temperature for a blend of such elastomers may exhibit one only maximum peak. Therefore, for rubber compositions based upon such on such compatible rubber blends, low temperature physical properties are typically sacrificed for high temperature physical properties of the rubber composition. For example, a tire treadwear property may be typically compromised with an increase in wet traction property when such compatible rubber blend is used for a tire tread rubber composition. Furthermore, the cold brittle point temperature of the tire tread rubber composition may typically be raised with an increase in wet traction when a compatible blend of such elastomers is used for the tire tread rubber composition to render the tire tread less useful at very low operating temperatures.
For a description of this invention, terms such as xe2x80x9ccompounded rubberxe2x80x9d, xe2x80x9crubber compoundxe2x80x9d and xe2x80x9ccompoundxe2x80x9d, if used herein, refer to rubber compositions composed of one or more elastomers blended with various ingredients, including curatives such as sulfur and cure accelerators. The terms xe2x80x9celastomerxe2x80x9d and xe2x80x9crubberxe2x80x9d might be used herein interchangeably. It is believed that all of such terms are well known to those having skill in such art.
A reference to glass transition temperature, or Tg, of an elastomer or elastomer composition, if referred to herein, represents the glass transition temperature(s) of the respective elastomer or elastomer composition in its uncured state or possibly a cured state in a case of an elastomer composition. A Tg can be suitably determined by a differential scanning calorimeter (DSC) at a temperature rate of increase of 10xc2x0 C. per minute. (ASTM 3418).
A reference to a rubber composition""s brittle point relates to a cold temperature property value according to ASTM D746 for the rubber composition and is a measure of a temperature when a rubber composition becomes brittle and therefore a measure of the usefulness of the rubber composition at very cold temperatures
In accordance with this invention, a pneumatic rubber tire is provided having a tire tread, (circumferential rubber tread) where said tread is a rubber composition comprised of, based on parts by weight per 100 parts by weight of the tread rubber elastomers (phr),
(A) elastomers comprised of
(1) about 5 to about 45, alternatively about 10 to about 40, phr of styrene/isoprene copolymer elastomer (SIR) having a styrene content in a range of from 30 to about 55, alternatively from 30 to about 50, percent, a single Tg within in a range of about 0xc2x0 C. to about xe2x88x9240xc2x0 C., alternately in a range of from about xe2x88x925xc2x0 C. to about xe2x88x9230xc2x0 C., and wherein the isoprene component of said SIR has a microstructure comprised of a maximum 3,4-polyisoprene content of 11 percent, and
wherein the styrene distribution in said SIR is characterized in that,
(a) the minimum percent of the styrene units present in not more than five sequential styrene units (St5) is equal to about the sum of 125.6 minus 0.92 times the percent by weight of styrene units in the SIR, and
(b) the maximum percent of styrene units present in more than five sequential styrene units is equal to 100 less the said minimum St5 unit percent, and
(2) about 55 to about 95, alternatively about 60 to about 90, phr of cis 1,4-polybutadiene rubber (BR) having a cis 1,4-content of at least 95 percent and a single Tg within in a range of from about xe2x88x92100xc2x0 C. to about xe2x88x92107xc2x0 C.,
wherein the weight ratio of said BR to said SIR is at least 1.1/1 and alternately in a range of from about 60/40 to about 80/20,
wherein the Tg of said BR is at least 60xc2x0 C., and preferably at least 85xc2x0 C., lower than the Tg of said SIR and optionally, at least 95xc2x0 C. lower than the Tg of said SIR;
(B) about 40 to about 110, alternatively about 50 to about 90, phr of reinforcing filler comprised of at least one of carbon black, aggregates of precipitated silica and silica-containing carbon black which contains silica domains on its surface;
(C) a coupling agent, when said reinforcing filler is precipitated silica and/or said silica-containing carbon black, having a moiety reactive with hydroxyl groups contained on the surface of said aggregates of precipitated silica and hydroxyl groups contained on the surface of the silica domains of said silica-containing carbon black.
In practice, said BR may be a specialized BR as a cis 1,4-polybutadiene elastomer having a cis 1,4-content in a range of about 96 to about 98 percent, and prepared by polymerizing 1,3-butadiene in the presence of organonickel compound as a nickel salt of a carboxylic acid, organoaluminum compound as a trialkylaluminum compound, hydrogen fluoride and para-styrenated diphenylamine, wherein said organoaluminum compound and said fluorine containing compound are brought together in the presence of said para-styrenated diphenylamine. Such specialized BR may have a Mooney, (ML 1+4) at 100xc2x0 C., viscosity value in its unvulcanized state in a range of about 35 to about 45 and a cold flow value of less than 1.2 mg/minute. Such specialized BR may also be referred to herein as a Branched BR as it is considered herein as containing a degree of branching. For the purposes of describing this invention, the term xe2x80x9cBRxe2x80x9d is usually intended to include the specialized BR, or branched BR, unless otherwise indicated.
In the further practice of the invention, said rubber composition may also contain from zero to about 20, alternatively about 10 to about 20 phr of at least one elastomer, other than said SIR and BR elastomers, selected from polymers of isoprene and/or 1,3-butadiene and copolymers of styrene with 1,3-butadiene and copolymers (terpolymers) of styrene, 1,3-butadene and isoprene.
A significant aspect of this invention is the use of a high styrene-containing styrene/butadiene elastomer (SIR) in combination with a BR, and particularly including said specialized BR, in which the BR is in the majority and therefore considered herein as being the continuous phase insofar as the said SIR and said BR elastomers are concerned as well as a requirement that said elastomers individually have single Tg""s within the prescribed temperature ranges and the Tg of said BR is at least 60xc2x0 C., preferably at least 85xc2x0 C., lower than said Tg of said SIR.
For a more detailed preparation of said specialized BR, considered herein to be a branched and more easily processible BR, U.S. Pat. No. 5,451,646 is hereby incorporated by reference in its entirety.
In general, said specialized BR may be characterized by having a cis 1,4-content of at least 96 percent, a Tg in a range of about 100xc2x0 C. to about 107xc2x0 C., a Mooney (ML 1+4) at 100xc2x0 C. viscosity value in a range of from about 30 to about 40 and preferably a melting point (ASTM D3418) in a range of about xe2x88x924xc2x0 C. to about xe2x88x926xc2x0 C.
The high styrene containing styrene/isoprene elastomer which contains a substantially random styrene distribution (SIR) may be prepared, for example, by the procedure described in Example I herein.
It is to be appreciated that said SIR elastomer contains its units derived from styrene in a substantially random distribution in said copolymer as presented by the aforesaid styrene distribution characterization in which a major portion of the styrene units are present in a sequence of 5 or less sequential units and only a minor portion are present in greater than 5 sequential units.
The difference in spatially defined Tg""s of the SIR and the BR elastomers of at least 60xc2x0 C., preferably at least 85xc2x0 C., is also considered to be an important aspect of the invention, in order to provide a good balance of both relatively good abrasion resistance indicative of tire treadwear for a tire tread of such rubber composition and relatively high hysteresis at low temperatures in a range of xe2x88x9220xc2x0 C. to +10xc2x0 C. (or even up to +20xc2x0 C.) as an indication of good wet traction of a tire tread of such rubber composition.
In this manner, it is considered herein that the relatively low Tg BR elastomer is relatively immiscible with the higher Tg SIR elastomer so that such elastomers are of two phases. By using BR in the majority, the BR phase is considered herein as being the continuous phase and the SIR being of a discontinuous phase, insofar as said two elastomers are concerned.
In the practice of the invention, it is important that the BR, and particularly the specialized, improved processing, cis 1,4-polybutadiene rubber (sometimes referred to herein as a specialized BR) is used in the majority and therefore is the continuous phase of the blend of the relatively immiscible BR and SIR elastomers. The use of the specialized, improved processing BR is considered herein to aid in the processing of the rubber composition which contains the specialized BR. By having the BR as the continuous phase, it is considered herein that reduction in tire treadwear and brittle point of the rubber composition are optimized. That is, treadwear and brittle point are controlled and dominated by the continuous phase, which for this invention, is the BR which is considered herein to be the desirable elastomer for tire treadwear and rubber composition brittle point considerations.
In practice, the relatively high styrene-containing styrene/isoprene copolymer elastomer (SIR), while used in the minority as compared to the BR elastomer, it is considered herein to be important to promote tire tread traction, hysteresis, or coefficient of friction, for the rubber composition. It is desired herein that the relatively high styrene content of the SIR elastomer be at least 30 percent to aid in promoting tire tread traction enhancement. By having the SIR as the minor, dispersed phase, it is considered herein that the rubber composition""s hysteresis and wet traction properties can be enhanced without adversely affecting the treadwear and brittle point properties.
Use of the specialized high cis 1,4-polybutadiene rubber (BR) is considered herein to be important in order to promote a good balance of abrasion resistance and hysteresis for the rubber composition and to aid in the processability of the rubber composition and, further, it allows for a use of a greater amount of a polybutadiene rubber in the rubber composition.
Use of a minor amount of cis 1,4-polyisoprene, preferably natural rubber in the rubber composition, is considered herein to be normally useful to enhance uncured rubber composition processability with minimum processing oil and processing additives which are considered herein to adversely offset abrasion resistance.
The addition of a small, minor amount of cis 1,4-polyisoprene natural rubber is also considered herein to be important to contribute to tear resistance property for the tread rubber composition.
Use of rubber reinforcing carbon black(s) and silica reinforcement fillers for this invention, is considered herein to be important in order to provide good abrasion resistance, or coefficient of friction and also enhanced, or relatively high hysteresis for relatively good traction for a tire tread.
Representative of such rubber reinforcing carbon blacks are, for example, N121 N205 and N299. Examples of reinforcing carbon blacks for elastomers, generally, together with their Iodine number values and DBP (dibutyl phthalate) absorption values, may be found in The Vanderbilt Rubber Handbook, (1990), 13th Edition, Pages 416 through 419.
As hereinbefore related, use of the combination of the aforesaid SIR (in the majority) and BR (in the minority) elastomer blend is considered herein to be important in order to optimize, for the tire tread, both abrasion resistance (to enhance treadwear) and hysteresis (to enhance traction).
Indeed, it is considered herein that the utilization of the selected and combined elastomers to provide an immiscible elastomer blend as defined by a sulfur cured elastomer composition having the two Tan delta peaks (in a plot of tan delta values versus temperature), is significant, is believed to be novel and is believed to be a departure from using typical blends of elastomers with one tan delta peak cured elastomer composition within a temperature range of about xe2x88x9290xc2x0 C. to about 10xc2x0 C., utilizing the defined elastomers having Tg""s spaced apart by at least 60xc2x0 C.
As hereinbefore discussed, in the further practice of this invention, while maintaining the aforesaid basic synthetic elastomer composition, it is envisioned herein that the tread rubber composition may contain a very minor amount of about 5 to about 15, or perhaps up to 20, phr of at least one additional diene based synthetic elastomer.
Such additional synthetic diene based elastomers can be, for example, cis 1,4-polyisoprene, high vinyl polybutadiene having a vinyl content of from 30 to about 90 percent, and 3,4 polyisoprene.
In practice, it is preferred that the elastomers utilized in the tread composition, including both the other and the additional elastomers, are exclusive of polymers and copolymers of isobutylene, including halogen modifications thereof.
As hereinbefore pointed out, the invention is based upon a novel combination of elastomers, namely said SIR (in the majority) and said BR (in the minority). This aspect of the invention is considered particularly important for creating a tire tread rubber composition with good abrasion properties coupled, also, with good traction, or coefficient of friction, or hysteresis, properties. This aspect of the invention is especially directed to its more narrowly defined aspects which is restated, in its narrower form.
While it is considered that a tire tread rubber is conventionally reinforced with a particulate carbon black reinforcing filler, it is recognized that, for some tire applications, aggregates of precipitated silica reinforcement may also be desired. In such case, about 5 to about 25 phr or, alternatively, from 25 to about 80 or from about 30 to about 80 phr of precipitated silica might be used wherein the overall total of carbon and precipitated silica is in a range of from 40 to about 110.
The commonly employed siliceous pigments used in rubber compounding applications are usually aggregates of precipitated siliceous pigments (referred to herein as silica).
The siliceous pigments employed may be precipitated silicas such as, for example, those obtained by the acidification of a soluble silicate, e.g., sodium silicate.
The BET surface area of the silica, as measured using nitrogen gas, may, for example, be in a range of about 50 to about 300, alternatively about 120 to about 200, square meters per gram. A BET method of measuring surface area is described in The Journal of the American Chemical Society, Volume 60, Page 304 (1930).
The silica may also have a dibutylphthalate (DBP) absorption value in a range of about 100 to about 400, and usually about 150 to about 300, cm3/100 g. Various commercially available silicas may be considered for use in this invention such as, for example, only and without limitation, silicas commercially available from PPG Industries under the Hi-Sil trademark with designations 210, 243, etc; silicas available from Rhodia, with designations of Zeosil 1165MP, Zeosil 165GR and silicas available from Degussa AG with designations such as VN2, VN3, Ultrasil 3370 and Ultrasil 7005 and silicas from J. M. Huber as Zeopol 8745, Zeopol 8755 and Zeopol 8715.
Silica-containing carbon blacks which contain silica domains on their surface may be prepared, for example, by co-fuming carbon black and silica.
When silica reinforcement is used for a rubber tire tread, the silica is conventionally used with a coupling agent, or what is sometimes referred to as a reinforcing agent.
Compounds having a moiety capable of reacting with both hydroxyl groups contained on the silica surface (e.g. silanol groups) and another moiety interactive with the diene-based rubber elastomer molecule in a manner to cause the silica to have a reinforcing effect on the rubber, many of which are generally known to those skilled in such art as coupling agents, or couplers, are often used. Such coupling agents, for example, may be premixed, or pre-reacted, with the silica particles or added to the rubber mix during the rubber/silica processing, or mixing, stage. If the coupling agent and silica are added separately to the rubber mix during the rubber/silica mixing, or processing stage, it is considered that the coupling agent then combines in situ with the silica.
In particular, such coupling agents may, for example, be composed of a silane which has a constituent component, or moiety, (the silane portion) capable of reacting with the hydroxyl groups contained on the silica surface and, also, a constituent component, or moiety, capable of interacting with the diene-based rubber, particularly a sulfur vulcanizable rubber which contains carbon-to-carbon double bonds, or unsaturation. In this manner, then the coupler acts as a connecting bridge between the silica and the rubber and thereby enhances the rubber reinforcement aspect of the silica.
In one aspect, the silane of the coupling agent apparently forms a bond to the silica surface, possibly through hydrolysis, and the rubber reactive component of the coupling agent combines with the rubber itself.
Numerous coupling agents are taught for use in combining silica and rubber such as, for example, silane coupling agents containing a polysulfide component, or structure, such as bis-(3-trialkoxysilylalkyl)polysulfide, such as for example, bis-(3-triethoxysilylpropyl)polysulfide, which contains an average of either 2 to 2.6 or from 3.5 to 4 connecting sulfur atoms in its polysulfidic bridge, preferably an average of from 2 to 2.6.
It is readily understood by those having skill in the art that the rubber compositions of the tread would be compounded with conventional compounding ingredients including the aforesaid reinforcing fillers such as carbon black, as hereinbefore defined, possibly an addition of a minor amount of precipitated silica, usually, in combination with a silica coupling agent, as well as antidegradant(s), processing oil as hereinbefore defined, stearic acid or a zinc stearate, zinc oxide, sulfur-contributing material(s) and vulcanization accelerator(s) as hereinbefore defined.
Such compounding of rubber is well known to those having skill in such art. Antidegradants are typically of the amine or phenolic type. While stearic acid is typically referred to as a rubber compounding ingredient, it may be pointed out that the ingredient itself is usually obtained and used as a mixture of organic acids primarily composed of stearic acid with at least one of oleic acid, linolenic acid and palmitolic and/or palmitic acid normally contained in the stearic acid as typically used. The mixture may contain minor amounts (less than about six weight percent) of myristic acid, arachidic acid and/or arachidonic acid. Such material or mixture is conventionally referred to in the rubber compounding art as stearic acid.
Where normal or typical rubber compounding amounts or ranges of amounts of such additives are used, they are not otherwise considered as a part of the invention. For example, some of the ingredients might be classified, in one aspect, as processing aids. Such processing aids may be, for example, waxes such as microcrystalline and paraffinic waxes typically used in a range of about 1 to 5 phr and often in a range of about 1 to about 3 phr; and resins, usually as tackifiers, such as, for example, synthetic hydrocarbon and natural resins typically used in a range of about 1 to 5 phr and often in a range of about 1 to about 3 phr. A curative might be classified as a combination of sulfur and sulfur cure accelerator(s) for the rubber compound (usually simply referred to as accelerator) or a sulfur donor/accelerator. In a sulfur and accelerator(s) curative, the amount of sulfur used is in a range of about 0.5 to about 5 phr and usually in a range of about 0.5 to about 3 phr; and the accelerator(s), often of the sulfenamide type, is (are) used in a range of about 0.5 to about 3 phr and often in a range of about 1 to about 2 phr. However, the preferred cure system of sulfur and accelerators has already hereinbefore been defined.
The ingredients, including the elastomers but exclusive of sulfur and accelerator curatives, are normally first mixed together in a series of at least two sequential mixing stages, although sometimes one mixing stage might be used, to a temperature in a range of about 145xc2x0 C. to about 180xc2x0 C., and such mixing stages are typically referred to as non-productive mixing stages. Thereafter, the sulfur and accelerators, and possibly one or more retarders and one or more antidegradants, are mixed therewith to a temperature of about 90xc2x0 C. to about 120xc2x0 C. and is typically referred as a productive mix stage. Such mixing procedure is well known to those having skill in such art.
After mixing, the compounded rubber can be fabricated such as, for example, by extrusion through a suitable die to form a tire tread. The tire tread is then typically built onto a sulfur curable tire carcass and the assembly thereof cured in a suitable mold under conditions of elevated temperature and pressure by methods well-known to those having skill in such art. In such case of retreading of a tire, the tire tread might first be precured and then applied to the already cured tire carcass with a curable gum strip between the tread and carcass and the assembly then submitted to curing conditions to cure the aforesaid gum strip.
The invention may be better understood by reference to the following example in which the parts and percentages are by weight unless otherwise indicated.